The present invention relates to a technique effective in forming on an IC chip a filter with a reduced capacitance to be provided on the output side of a direct downconversion mixer which, for frequency conversion, combines a reception signal and an oscillator signal of a prescribed frequency. More particularly, the invention relates to a technique which can be effectively applied, for example, to a semiconductor integrated circuit (radio frequency IC) for processing radio frequency signals.
In a radio communication system such as a portable telephone, a radio frequency IC (RF IC) incorporating a mixer which, for frequency conversion, combines a reception signal and an oscillator signal of a prescribed frequency is used. Conventionally proposed portable telephones include dual-band portable telephones capable of processing signals on two frequency bands, for example, the 925 to 960 MHz GSM (Global System for Mobile Communication) and the 1,805 to 1,880 MHz DCS (Digital Cellular System). In recent years, there is also a demand for triple-band portable telephones which can process signals, for example, on the 1,930 to 1,990 MHz PCS (Personal Communication System) in addition to those on the GSM and DCS. It is considered that portable telephones compatible with more bands will come into demand in the future.
For an RF IC for use in a portable telephone compatible with plural bands, a direct conversion type is effective from a viewpoint of reducing the number of components. The output of a mixer which, for frequency conversion, combines a reception signal and an oscillator signal of a prescribed frequency contains a signal component of a frequency equivalent to a frequency difference between the two signals and signal components of other frequencies. In the receiver circuit including the mixer, a low-pass filter for removing unwanted waves and signals of adjacent channels is provided on the output side of the mixer.
The output signal of a direct conversion mixer is required to have a frequency of several hundred kHz. Therefore, the cutoff frequency of the low-pass filter provided on the output side of the mixer is required to be several hundred kHz. To realize such a cutoff frequency, it is necessary to use a capacitive element having a very large capacitance, that is, 1,000 pF or more. Such a requirement used to be met by using an external capacitive element. A technique in which an external element is used as a capacitive element for inclusion in a low-pass filter to be provided on the output side of a Gilbert cell mixer is disclosed in Japanese Unexamined Patent Publication No. 2004-104040.